Hello,

i am interested in the band gaps of transition metal compounds, i understand the description of mott-hubbard and zaanen and sawatzky for partially filled transition metal compounds.

However i think about three points, in which i cannot understand this theory:

1.) It is not distinguished between partially filled t2g and eg band, why?

Example: if a compound has a half-filled t2g band and high Udd and CT it is an insulator.

However if the band gap (t2g to eg) is lower, the electron transport can happen via excitation to eg, so MH and CT would be irrelevant.

2.) Classification into Mott-hubbard and Charge transfer insulators is done for TM oxide with filled bands (LiCoO2 and TiO in Low spin state). How is charge fluctuation possible in this materials?